Engine oils function under severe oxidative conditions. The oxidative breakdown of the engine oil creates sludge and deposits, deteriorates the viscosity characteristics of the oil, and produces acidic bodies that corrode engine parts. To combat the effects of oxidation, engine oils are formulated with an array of antioxidants including hindered phenols, aromatic amines, zinc dithiophosphates (ZDDP), sulfurized hydrocarbons, metal and ashless dithiocarbamates, and organo-molybdenum compounds. Particularly effective antioxidants are alkylated diphenylamines (ADPAs), and ZDDPs. In combination, these two compounds provide the majority the of the antioxidant capacity in engine oils under current practice. However, the use of ZDDP in engine oils is declining due to the poisoning effect of phosphorus on exhaust after-treatment catalyst. In addition, sulfur levels in engine oils are also in decline due to the effect of sulfated ash exhaust after-treatments. Thus, a need exists for effective antioxidant chemistry that can reduce or eliminate the need for phosphorus and sulfur containing antioxidants.
Additionally, organomolybdenum compounds have been used as a component in engine oils as an antioxidant. However, because of the high costs associated with the metal and the impact this cost has on the treat levels and overall cost of the additive packages, there has been an interest within the industry to lessen the dependence on molybdenum based antioxidants. Besides cost however, molybdenum presents problems or concerns with respect to copper/lead bearing corrosion, rust inhibition and particularly with the ball rust test that is part of the GF-4 specification for engine oils. Still further is the concern with respect to the TEOST 33 procedure being proposed for GF-5. That test looks at deposit control under high temperatures and exposure to NOx environments. It has been found that with Mo levels higher than 350 ppm, high levels of deposits are formed, which makes it difficult to formulate an oil that will pass the proposed GF-5 specification. Until now, however, suitable formulations which can obtain the benefits of molybdenum, while avoiding its use, have not been found.
Mono-esters of glycerol or monoglycerides, ethoxylated amides, and borate esters thereof have long been recognize as effective friction reducing and antiwear additives for lubricants as disclosed by U.S. Pat. Nos. 4,389,322, 4,450,771, 5,629,272, which are hereby incorporated herein for reference. Furthermore, these materials have been combined with friction reducing molybdenum compounds to further improve friction reducing capacity of engine oils and thus, improving fuel economy of passenger cars.
In U.S. Pat. No. 6,723,685 B2, Hartley discloses a lubricating oil composition comprising a) an oil of lubricating viscosity having viscosity index of least 96; b) a least one calcium detergent; c) at least organic molybdenum compound; d) at least one organic ashless nitrogen free friction modifier; and e) at least one metal dihydrocarbyldithiophosphate compound, the composition having molybdenum in the amount of least 10 ppm and phosphorus form metal dihydrocarbyldithiophosphate compound in amount up to about 0.1 wt. %. The preferred organic ashless nitrogen is mono-ester of glycerol in which the ester is an oleate. Hartley states that this composition is very effective in reducing fuel consumption in modified Sequence VIB engine test without having detrimental effect of elastomer seals. However, Hartley does not teach that glycerol mono-oleate has antioxidant or deposit control function.
In U.S. Patent Application 2006/0025313 A1, Boffa discloses a low phosphorus lubricating oil composition for internal combustion engines which demonstrates fuel economy benefits while also providing high temperature oxidation, piston deposits, and wear protection. The lubricating oil composition of this invention is comprised of a) major amount of base oil of lubricating viscosity; b) overbased alkaline earth sulfonate detergent, c) 0.02 to 10 wt % of a oxymolybdenum-containing complex, d) from 0.1 to 5 wt % of ester friction modifier; and e) from about 0.2 to 10 wt % of antioxidant selected from group consisting of a diphenylamine type, a sulfurized ester-containing compound and mixtures thereof, wherein phosphorus content of the composition is 0.08 wt % or less. In this invention, the preferred ester friction modifier is borated glycerol mono-oleate. However, Boffa does not teach that ester friction modifier has antioxidant function and relies on both diphenylamine type antioxidants and molybdenum complex to provide oxidation and deposit control protection.
It has been shown that a combination of an alkylated diphenylamine and borated monoglyceride/ethoxylated amide exhibits antioxidant synergy in a lubricating composition. However, this combination alone is not sufficient to provide adequate antioxidant protection where lower levels of phosphorus are used.
The invention herein discloses low phosphorus or phosphorus-free, molybdenum-free lubricating compositions that achieve the same level of oxidation stability as compositions with higher level of phosphorus. By molybdenum-free, it is meant that the lubricating composition is free of molybdenum in any form, whether elemental, or as part of a bound or unbound ion, or as part of an inorganic or organic molecule or complex.